In modern society with advanced information technology, the importance of mobile electronic devices such as smartphones and notebook PCs has been rapidly increasing. This has also inevitably increased the importance of secondary batteries as power sources. Among others, lithium secondary batteries have been used in most of the mobile electronic devices currently available on the market for reasons such as their high energy density.
Moreover, in recent years, with an increasing concern about the exhaustion of fossil fuels and environmental issues, attempts have been actively made to use a lithium secondary battery as the power source of an automobile, as represented by an electric vehicle (EV), a plug-in hybrid electric vehicle (PHEV), etc. A secondary battery to be installed in an automobile is required to have a better battery characteristic as compared with a secondary battery for a mobile electronic device. Particularly, there is a demand for improving the high-rate characteristic, which contributes to the acceleration of the automobile, and the energy density, which contributes to the distance to be covered. Therefore, there is also a demand for a nonaqueous electrolyte solution used in a lithium secondary battery for an automobile to have better characteristics.
As an organic solvent of a nonaqueous electrolyte solution for a lithium secondary battery, various solvents have been proposed in the art, including a cyclic carbonate such as ethylene carbonate and propylene carbonate, a chain carbonate such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, etc. Typically, in many cases, different ones of these cyclic carbonates and chain carbonates are mixed together at an appropriate proportion to be used as a solvent of a nonaqueous electrolyte solution.
The following two are the primary properties that are demanded as a nonaqueous electrolyte solution in order to improve the high-rate characteristic of a lithium secondary battery.
1. Ionic conductivity of nonaqueous electrolyte solution is high.
2. Interface resistance at interface between electrode and nonaqueous electrolyte solution is low.
As a specific attempt for enhancing the ionic conductivity of the nonaqueous electrolyte solution, Patent Document No. 1, for example, discloses adding 1 to 10% of a low-viscosity nitrile solvent to a mixed solvent of ethylene carbonate, vinylene carbonate and a chain carbonate. As an attempt to reduce the interface resistance, Patent Document No. 2, for example, discloses that using a silyl borate-based compound and an acid anhydride in the nonaqueous electrolyte solution improves the characteristic of the interface between the negative electrode and the nonaqueous electrolyte solution, and improves the high-rate characteristic of the battery. On the other hand, the nonaqueous electrolyte solution does not directly contribute to the improvement of the energy density of the lithium secondary battery. However, it is important to improve the characteristic of the nonaqueous electrolyte solution in order to ensure a long-term reliability of a lithium secondary battery having a high energy density.
In order to improve the energy density of a lithium secondary battery, it is effective to use a negative electrode having a lower operating potential and a positive electrode having a higher operating potential, thereby realizing a secondary battery of a high voltage specification. In such a case, the nonaqueous electrolyte solution need to be stably present in the voltage range, and for the long-term reliability of the lithium secondary battery, there are issues such as the reductive degradation of the nonaqueous electrolyte solution at the lower-potential negative electrode and the oxidative degradation of the nonaqueous electrolyte solution at the higher-potential positive electrode.
A carbon material such as graphite has been widely used as a negative electrode material having a low operating potential, and a method of forming a stable coating, referred to as SEI (Solid Electrolyte Interface), on the negative electrode surface has been generally known as a method for suppressing the reductive degradation of the nonaqueous electrolyte solution at the negative electrode. Compounds generally known to have the ability to form an SEI include ethylene carbonate, vinylene carbonate, 1,3-propanesultone, vinyl ethylene carbonate, 1,3-propanesultone, butanesultone, etc. Ethylene carbonate is often included as a high-dielectric-constant solvent by about 10 to 50% in the electrolyte solution of a lithium secondary battery. The other compounds listed above are typically included as an electrolyte solution additive by about 0.1 to 5% in the electrolyte solution. The inclusion of such an SEI-forming compound in the nonaqueous electrolyte solution suppresses continuous reductive degradation of the nonaqueous electrolyte solution, and it is therefore possible to improve the long-term reliability of the secondary battery.
A method of using fluoroethylene carbonate as a cyclic carbonate of the solvent is generally known as a method for improving the oxidation resistance at the positive electrode. As compared with ethylene carbonate, which is typically used in a lithium secondary battery used with a charging voltage of 4.2 V or less, fluoroethylene carbonate has a fluoro group introduced therein having a high electron-withdrawing property, and it is therefore possible to improve the oxidation resistance. Thus, it is known to improve the long-term reliability of a lithium secondary battery used with a charging voltage of 4.3 V or more. Although the introduction of a fluoro group lowers the reduction resistance, it is also known in the art that fluoroethylene carbonate undergoes reductive degradation at the negative electrode surface, thereby forming a stable SEI and suppressing continuous reductive degradation.
Thus, in a lithium secondary battery used with a charging voltage of 4.3 V or more, using a cyclic carbonate having a fluoro group, such as fluoroethylene carbonate, achieves a certain level of an advantageous effect in terms of ensuring a long-term reliability.